The coating of thin films on the surfaces of parts or elements, and the provision of predetermined functions, is often conducted in an extremely wide range of fields, such as electric and electronic parts, tools, machine parts, and the like. Semiconductor devices such as integrated circuits and the like are representative thereof; such devices are produced by repeated film formation and minute working, so that film forming apparatuses play an extremely important role in the manufacturing processes thereof.
Furthermore, in integrated circuits, in concert with the miniaturization of the devices, higher performance and quality are required, and for this reason, there has been a strong demand for a film forming method which is capable of forming, according to design, the impurity distribution in deposition films comprising the devices. For example, in the case of a bipolar transistor, the high speed characteristics of LSI, such as high speed operating elements, memory, digital/analog converters, and the like, or of elements which are extremely widely employed as discrete elements, are determined by how fast electrons pass through the base layer, so that a structure in which the base layer is made thinner and the passage distance is reduced, a gradient is created in the impurity concentration in the thin base layer and an internal electric field is formed, and which is capable of accelerating the electrons and exhibits a sharp impurity density change at the interface, for example, an impurity density distribution having a gradient such as that shown in FIG. 9, is effective in increasing element performance.
Conventional film forming apparatuses can be broadly divided into two groups based on the operating principle of the film forming method. That is to say, such apparatuses can be classified into film forming apparatuses which employ physical methods such as vapor deposition, sputtering, and the like, and film forming apparatuses which employ chemical reactions of film forming raw material gases, or the so-called chemical vapor deposition (CVD) methods. Such apparatuses have superior operability, simplicity of maintenance, high operation rates, and the stability of the quality is also superior, so that such apparatuses are widely employed; however, it is difficult to change the impurity density of the deposition film described above or the component ratio, and furthermore, it is difficult to form sharp changes in impurity density.
For example, when the impurity density of the deposition film or the component ratio is changed by means of vapor deposition or sputtering, it is necessary to change the impurity density or component ratio of the vapor deposition materials or the target materials themselves in correspondence with the impurity density of the deposited film or the component ratio. Furthermore, when aluminum silicide used as the wiring material of the semiconductor integrated circuit is formed by means of sputtering, it is necessary that the target have the precisely appropriate combination of aluminum and silicon. In addition, when deposition films having differing component ratios are formed, it is necessary to change the target each time to one which is appropriate for the composition of the deposition film. However, the impurity density or component ratio in the direction of depth of films deposited using standard film forming apparatuses is as shown in FIG. 8(a), and is constant in the direction of depth.
When attempts are made to control the impurity density or component ratio in the direction of depth as, for example, in FIG. 8(b), as a continuous function of the direction of depth (in the Figure, a straight-line relationship), such control is extremely difficult using vapor deposition or sputtering methods, so that as an approximating measure, as shown in FIG. 8(c), it is necessary to divide the depth into a number of regions n (in the Figure, n=4), and to change the impurity density or component ratio in a stepped manner. In order to do this, it is necessary to prepare a number of appropriate vapor deposition materials or targets in advance, as described above, and to interchange these at standard film thicknesses and thus to conduct film formation; however, as the number of divisions increases, the process becomes cumbersome, and it is thus not a practical method.
Furthermore, attempts have been made using the CVD method to form impurity density distributions within deposition films by means of continuously changing the ratio of the flow rates of the raw material gases during film deposition; however, in the case for example in which a p-type silicon layer is formed, the B.sub.2 H.sub.6 gas is normally diluted to within a range of 100 ppm-2% in order to maintain the stability thereof, so that in order to alter the impurity density in the deposition film, it is necessary to greatly alter the ratio of the flow rate thereof with that of the SiH.sub.4 gas, and as a result (the total flow rate changes) the discharge state changes, and the quality of the deposition film in the direction of layer thickness changes in an undesirable manner.
Additionally, in order to form a high-quality deposition film using a conventional CVD method, it was necessary to conduct film formation, for example in the case of monocrystalline silicon, at a high temperature within a range of 900.degree.-1100.degree. C., and furthermore, even in the case of polycrystalline silicon, the film was formed at a temperature within a range of 550.degree.-600.degree. C. by means of a reduced-pressure CVD method, so that a redistribution of the impurity atoms occurred as a result of thermal dispersion, and it is currently the case that the desired impurity density distribution cannot be obtained. Furthermore, attempts have been made to form the deposition film by means of a PCVD method; however, in order to form a high-quality film, annealing is necessary at a temperature of approximately 700.degree. C., and this causes a problem in that a redistribution of the impurity atoms is caused.
As described above using conventional film forming methods and film forming apparatuses, it is currently extremely difficult to accurately control the density and composition in the direction of depth of the deposition film while maintaining the quality of the deposition film.
In view of the above circumstances, it is an object of the present invention to provide a film forming method and a film forming apparatus which are capable of continuously changing the density or composition in the direction of film thickness of a deposition film, and which are capable of obtaining a high quality film in a stable manner.